There is known WAVE (Wireless Access in Vehicular Environments) as a telecommunications standard for a communication device managed by a service provider (hereinafter, service provision device) and communication devices used in mobile objects such as vehicles (hereinafter, in-mobile devices) to execute direct wireless communications with each other, e.g., in Patent literature 1.
The above service provider signifies an entity such as company, shop, public institution, which provides a predetermined service to users using the in-mobile devices. For convenience, hereinafter, the system in which the service provision device and the in-mobile devices communicate with each other in compliance with the WAVE standard is referred to as a mobile communication system.
WAVE specifies two kinds of communication channels of a control channel (CCH, hereinafter) and a service channel (SCH, hereinafter); two kinds of communication channels are used for communication devices to perform communications. An SCH is a communication channel used to transmit and receive the information for providing and using services. A CCH is a communication channel used to distribute the message (hereinafter, WSA: Wave Service Advertisement) for the service provision device to start the communication with the in-mobile devices using the SCH. The in-vehicle device can recognize a currently available service by receiving the WSA transmitted from the service provision device via the control channel.
Further, WAVE designates alternately a CCH time zone in which the communication using the CCH should be performed and an SCH time zone in which the communication using the SCH may be allowed, with predetermined time intervals (each 50 milliseconds, for instance). The in-mobile device switches a communication channel as a target for transmission and reception (hereinafter, communication-target channel) depending on the transitions of the time zones, thereby performing the communication via the control channel or the communication via the service channel.
For example, the in-mobile device designates the CCH as a communication channel when not needing to perform the communication using an SCH. By contrast, when needing to perform the communication via an SCH, the communication-target channel is switched from the CCH into the SCH at the switching point of time at which the CCH time zone transitions into the SCH time zone, thereby performing the communication using the SCH. Then, at the point of time at which the SCH time zone is completed, the communication-target channel is returned from the SCH to the CCH. Note that the case where the in-mobile device needs to perform the communication using the SCH includes the case where after receiving a WSA transmitted using the CCH from a service provision device, the in-mobile device uses the service indicated in the WSA.